media: cedrus: add action item to the TODO

Mention that the request validation should increment the memory refcount
of reference buffers so we don't forget to do this.

Acked-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>